Existing power generation systems, such as with thermal power generation emitting a large amount of greenhouse gases and environmental pollutants by using fossil fuels, and with nuclear power plants entailing their stability issues and hazardous waste processing, have come to various destined breaking points. In response thereto, research efforts have increased significantly to develop environmentally friendlier, higher efficiency energy sources, and a power supply system using the same.
In particular, power storage technology has been the focus of research and development activities for broadening the usability of renewable energy sources against their significant susceptibility to external conditions and for enhancing the efficiency of power utilization, wherein secondary batteries receive more intensive interest and their research and development efforts are actively made.
A redox flow battery refers to an oxidation/reduction cell capable of converting chemical energy of an active substance directly into electrical energy, and it represents an energy storage system adapted to store new and renewable energy with large output variations according to environmental conditions such as sunlight and wind, and to convert the same into high-quality power.
Specifically, the redox flow battery has electrolytes containing an active material that causes an oxidation/reduction reaction, and that circulate between opposite electrodes and a storage tank, to perform charging and discharging.
Such a redox flow battery typically includes tanks containing active materials in different oxidized states, a pump for circulating the active materials during charge/discharge, and unit cells partitioned by a separation membrane, wherein the unit cell includes electrodes, an electrolyte, a current collector, and a separation membrane.
The electrolyte includes an active material that undergoes the oxidation/reduction process for enabling the charge/discharge operation, serving as an important factor to determine the battery capacity.
For example, a vanadium flow battery has an electrolyte solution composed of four kinds of ions with different oxidation numbers.
When driving the redox flow battery, a difference in reaction rates between an anode and a cathode may lead to an imbalance of concentration of metal ions (for example, vanadium ions), which causes a concentration difference and a volume difference between the electrodes.
When the balance between the electrolyte solutions collapses, there is a problem in that the absolute amount of redox pairs that can substantially participate in reaction is reduced, so the performance and charge retention rate of a battery are reduced, and electrolyte solutions should be replaced.
In order to solve the above problem, conventionally, methods of applying a predetermined additive to an electrolyte have been known.
For example, Korean Unexamined Patent Publication No. 2012-0132620 discloses a method for improving energy density of a redox flow battery in which the charging state of the redox flow battery is enhanced using an electrolyte solution containing metal ions, such as manganese ions, lead ions, cerium ions, or cobalt ions, so as to increase the usage rate of vanadium (V) ions in the electrolyte solution, thereby improving the energy density of the redox flow battery.